A power MOSFET typically comprises a large number of MOSFET transistor cells all connected in parallel with one another, i.e. with their gates connected together, their drains connected together, and their sources connected together. In order to facilitate sensing a load current of a power MOSFET when it is conductive or switched on, without requiring a series resistor to pass all of the load current and hence have a significant and undesired power dissipation, it is known to provide a current sensing MOSFET in which a relatively small number of the transistor cells (referred to as the current mirror section) of the MOSFET have a separate source terminal, referred to as a mirror or sense terminal. A current sensing resistor can be connected between the mirror and source terminals to pass only a predetermined small fraction of the total load current, so that power dissipation in the current sensing resistor is relatively small.
ON Semiconductor application notes AND8093/D entitled “Current Sensing Power MOSFETs”, July 2002 and AND8140/D entitled “SMART HotPlug™ Current Limit Function”, November 2004 describe some aspects of current sensing power MOSFETs, and refer to such a MOSFET as a SenseFET™. ON Semiconductor publication NIS5101/D, entitled “SMART HotPlug™ IC/Inrush Limiter/Circuit Breaker”, February 2005 describes an integrated circuit that combines a current sensing MOSFET with a control circuit. Philips Semiconductors application note AN10322—1 entitled “Current Sensing Power MOSFETs”, September 2004, also describes aspects of current sensing power MOSFETs.
The AND8093/D and AN10322—1 references describe using a virtual ground sensing circuit, which enables the gate-source voltages of the current mirror and main sections of the MOSFET to be equal so that accurate current sensing can be achieved. However, virtual ground current sensing has the significant disadvantages of requiring a relatively negative power supply and producing a relatively negative output voltage.
These references also describe the simpler technique of sense resistor current sensing, which does not require a negative power supply. As described in AND8093/D, it is desirable for the resistance of the sense resistor to be less than about 10% of the on-resistance of the current mirror section of the MOSFET, so that the sensed current is approximately equal to the load current divided by the current mirror ratio of the MOSFET. In practice, larger values of resistance may be needed to provide sufficient voltage drop for accurate sensing. As a larger resistance value appreciably increases the total resistance in the current mirror path, it makes the gate-source voltage of the current mirror section different from that of the main section of the MOSFET, leading to inaccurate current sensing.
This can be allowed for using equations provided in AND8093/D using the typical connection arrangement shown in FIG. 2 of that publication or in FIG. 1 of AND8140/D. However, such equations rely on resistance parameters of the MOSFET that may not be accurately known and that may vary with temperature. Consequently, a load current determined in this manner may not meet a desired accuracy. In addition, it may be desired to monitor the sensed voltage drop, for example supplying it to an analog-to-digital converter (ADC) to provide a digital value. In this case, an inconvenient and undesirable correction may be required at the output of the ADC.
There is therefore a need to provide improved current sensing arrangements for use with current sensing MOSFETs.